Measuring systems for measuring absorption or scattering of a medium are applied today in a large number of industrial applications, especially in chemistry and biochemistry as well as in water analysis, both for measuring taken samples as well as for on-line measurement of scattering or absorption. In measuring samples, the measuring chamber regularly comprises a cuvette fillable with a sample of the medium; the measuring chamber is inserted in the measuring system and is irradiated through correspondingly placed windows in the measuring system. In measuring on-line, the measuring system is embodied, for example, as a probe, which is placed in the medium. Here the measuring chamber is formed by a cavity in the probe, which is filled by the medium, and is irradiated through windows mounted on the external sides of the cavity.
In both absorption measuring as well as in scattering measuring, a transmitting unit is applied, which radiates light in a predetermined transmission direction into the measuring chamber through an entrance surface, and a detector is provided, which measures a radiation intensity, which is dependent on the absorption or scattering, emerging from the measuring chamber through an exit surface. Absorption and scattering measurement systems only differ by the number and positioning of the detectors. In absorption measuring, measurements are made in the transmission direction, i.e. only one detector is required, which is arranged downstream from an exit surface, in the main transmission direction, where the exit surfaces lies opposite the entrance surface.
In contrast, in scattering measuring, measurements are made at one or a plurality of predetermined scattering angles from the predetermined transmission direction. Here the detectors are located downstream from exit surfaces arranged at predetermined scattering angles from the transmission direction.
Described in DE 41 06 042 A1 is a measuring system for measuring low light absorptions at a single predetermined wavelength, subsequently to be referenced herein as the measuring wavelength. For this, a monochromatic measuring beam, whose light has the predetermined measuring wavelength, is sent through the medium. Additionally, a monochromatic reference beam is sent through the medium; the reference beam, which has a wavelength subsequently referred to herein as the reference wavelength, which has a value, for which the medium is transparent. In order to be able to measure small absorptions, here both a measurement radiation intensity, which has the measuring wavelength, emerging from the medium as well as a reference radiation intensity, which has the reference wavelength, emerging from the medium are measured, and the associated measurement and reference signals representing the measuring and reference radiation intensity are subjected to a direct, analog subtraction. In such case, a reference measurement is executed on a reference medium in advance; with the reference measurement, an intensity difference related to the measuring system between the measuring radiation intensity and the reference radiation intensity is first determined based on the difference. This intensity difference is compensated in the following measurement operation optically using a tunable gray filter or electronically using a corresponding weighting in the subtraction. In the following measurement operation, a quick, exact measuring of small absorptions is executed based on the difference between the measurement signal and reference signal automatically considering the intensity difference related to the measuring system. In this way, the measuring resolution is increased, since the resolution range comprises here only the order of magnitude of the difference, but not the essentially larger order of magnitude of the individual variables.
For this, the measuring beam and reference beam are sent simultaneously through the medium, and the total radiation emerging from the medium is fed via a filter matched to the measuring wavelength to a first detector, which measures the measurement radiation intensity of the measuring wavelength penetrating through the medium, and is fed via a second filter matched to the reference wavelength to a second detector, which measures the reference radiation intensity of the reference wavelength penetrating through the medium.
Alternatively, the production of a measurement beam and reference beam of different wavelengths by two different laser diodes is described; the measurement beam and reference beam are controlled mechanically by a chopper installed in the beam path or controlled electrically by an alternating current operation of the laser diodes; the measurement beam and reference beam are sent through the medium in rapid alternation. Another embodiment provides alternately clocking the measurement and reference beams with a first frequency and supplementally the reference beam with a second frequency by means of a chopper. In both cases all radiation intensities can be registered with a single detector, and therefrom the measurement radiation intensity and the reference radiation intensity are determined based on the two frequencies with which the measurement beam and reference beam are clocked one after the other by means of lock-in amplification technology.
There are a large number of applications, in which measurements with different wavelengths are required. An example of this is the measurement of absorption in the optical region, where, for example, color changes of reagent solutions are detected or monitored.
Current measuring systems applied for this comprise light sources, especially LEDs, of different wavelength, in which each is individually operated successively one after the other for a predetermined measurement duration. This offers the advantage that only one measuring detector is required for measuring the radiation intensity striking thereon, and a spectrometric splitting and analysis of the radiation emerging from the measuring chamber as well as an optical filter can be omitted. An example of this is described in U.S. Pat. No. 3,910,701.
Relatively large intensity jumps occur on the detector side by successively turning the individual light sources on and off; the intensity jumps are difficult to process both by the detector, e.g. a photodiode, as well as electronics following the detector, especially amplifier circuits for the amplification of the measurement signal of the detector. Since the amplifier circuit must be able very rapidly to accommodate the amplitude jumps of the detector signal, a high quality, fast amplifier is required. Fast amplifiers are, however, as a rule, very sensitive to electromagnetic disturbing influences, such as e.g. disturbance fields caused by motors or switching controllers. Due to the sensitivity of the amplifier, a crosstalk of electromagnetic disturbance fields from the transmitter to the amplifier circuit can also occur.
Moreover, hard switching events produce harmonic waves, which unavoidably occur in turning the operated light sources on and off one after the other; the harmonic waves can only be filtered out from the detector signal with difficulty.
A difference building between radiation intensities measured at different wavelengths following one another, as applied in the state of the art mentioned above, where the measuring occurs based on the difference between the measurement and reference signal, would indeed be suitable to reduce disturbances produced by switching events under certain circumstances; however, it cannot be applied here since the absolute values of the successively measured radiation intensities of the individual wavelengths are required.
Correspondingly, switching events are preferably reduced to a minimum, in that the duration of transmission times, in which one of the light sources transmits, is selected to be as large as possible. Transmission time durations of a tenth of a second and more are, consequently, no rarity. This is, however, especially problematic in measuring systems with many light sources of different wavelength, since a measuring cycle, in which the absorption or scattering of the individual different wavelengths is successively measured one after the other, requires a very long time. With ten different wavelengths, the duration of a complete measuring cycle would already be 1 second. Disturbances, such as e.g. air bubbles arising in the ray path for a short time, dust particles or other types of impurities in the medium, occurring for a short time in the medium correspondingly influence only the active partial measurements of the measuring cycle during the occurrence of these disturbances. In such case, each partial measurement affected by the disturbances arising in the medium for a short time, depending on the type of disturbance, measures a much too high or much too low radiation intensity. This can lead to drastically defective measurements. Absorption measurements for monitoring a color change of a medium, e.g. from red to green, are an example of this. If an air bubble occurs in the medium during the absorption measurement with the red light source and has already disappeared in the following measuring with the green light source, then a statement concerning the color change can no longer be made based on the ratio of the red transmission intensity to the green transmission intensity.
In measuring systems, which permanently operate a single monochromatic or multi-colored light source and measure the total radiation intensity penetrating through the medium, disturbances occurring in the medium for a short time can subsequently be recognized based on the associated sudden rises or declines in the measured radiation intensity, and at least the measurement results achieved in these periods of time are discarded. This is practically no longer possible with successive measurements with different wavelengths since a disturbance occurring in the medium for a short time here only affects individual measurement portions.
A shortening of the transmission times of the successively operated light sources would effect an improvement here in two ways. On the one hand, the period of time is shortened in which the short time disturbance has a disadvantageous affect. With long transmission times, short time disturbances, whose duration is shorter than the transmission time, corrupt the integral measurement over the entire transmission time interval. The shorter the transmission time is, the shorter the lasting negative influence of the disturbance over the duration of the disturbance. On the other hand, short time disturbances, which last clearly longer than the short transmission times, affect a number of partial measurements following one another and can therefore be subsequently recognized more easily.